What is the purpose of the "Color Space" on image texture nodes?
Depending on your color management setting in the Scene Properties Editor, Blender will interpret the colors in an image in different ways, (i.e. apply a gamma curve to it, or adjust the color gamut/saturation of the image^)

Changing the color space to noncolor basically tells Blender to skip that conversion, i.e. a bump map is should not be color-managed.

^ I am greatly simplifying what 'color management' is, but for the purpose of this, I think it's sufficient.